Light gas distillation processes are often characterized by the need to separate one or more components of a feed stream at elevated pressure. After substantial cooling and/or expansion, components contained within the feed stream can be separated within one or more distillation columns at cryogenic temperatures. Cryogenic air rectification and nitrogen rejection from natural gas are examples of such light gas distillation processes.
In a distillation process that is used in connection with the cryogenic rectification of oxygen and nitrogen containing streams, nitrogen is separated from oxygen. Where the feed stream is air, other components of air, such as argon, can also be separated. In such a process, a feed stream is compressed and then purified of higher boiling contaminants such as carbon dioxide, moisture and hydrocarbons. The resulting compressed and purified feed stream can be cooled within a main heat exchanger to a temperature suitable for its rectification and then introduced into a distillation column unit having a higher pressure column and a lower pressure column. The higher pressure column is thermally linked to the lower pressure column by a condenser-reboiler that can be in the base of the lower pressure column.
The feed is distilled within the higher pressure column to produce a nitrogen-rich vapor overhead and a crude-liquid oxygen bottoms. The nitrogen-rich vapor overhead can be condensed within the condenser-reboiler against boiling an oxygen-rich liquid that is collected in the base of the lower pressure column. The resulting nitrogen-rich liquid is used to reflux both the higher pressure column and the lower pressure column. The crude-liquid oxygen bottoms is introduced into the lower pressure column for further refinement. Oxygen and nitrogen product streams composed of a second nitrogen-rich vapor overhead and further oxygen-enriched liquid bottoms are extracted and can be introduced into the main heat exchanger and fully warmed in order to cool the incoming feed.
Distillation methods and apparatus can have other uses, for instance, in a nitrogen reject unit that is used for the separation and recovery of nitrogen from a hydrocarbon containing gas stream that can enter the distillation apparatus at pressure from a pipeline. Such a stream contains nitrogen that can be separated and returned for use in enhanced oil recovery projects. Commonly, an integrated dual distillation column arrangement is also used in which the higher pressure column is used to separate nitrogen from methane contained within the feed. A lower pressure column produces the nitrogen product.
In most cryogenic rectification systems, refrigeration must be supplied in order to offset ambient heat leakage, to facilitate heat exchanger operation and to produce liquefied products. In cryogenic air distillation feed air is compressed in a main air compressor and then purified. In instances where a product fraction is desired at substantial pressure, part of the feed air may be fully cooled, liquefied and a portion of which may be introduced into the higher pressure column. In instances where a substantial fraction of the air is desired as liquefied product, a second portion of the feed stream is introduced into a turboexpander to produce an exhaust stream that is also introduced into the higher pressure column. The part of the stream to be expanded can be further compressed within a booster compressor before being introduced into the turboexpander.
There often exists the need to recover liquid products from a cryogenic plant. The amount of liquid recovery is dependent upon the amount of refrigeration imparted to the plant. The turboexpander supplies such refrigeration. However, such turboexpanders represent a significant cost in equipment capital. As such, motivation exists to obtain the greatest liquid product flow from any given turboexpander. The refrigeration output of a turboexpander is dependent upon the expansion flow, pressure-expansion ratio and operating temperature.
In many instances, the only practical method of increasing the refrigeration output is by increasing the operating temperature at which the expansion occurs. However, there exists a limit to which this can be done particularly when the turbine exhausts directly into a column. As the turbine exhaust becomes superheated, its introduction into a distillation column will invariably result in the partial vaporization of down coming liquid. Packed columns are particularly susceptible to mal-performance from vapor feed superheating given the limited liquid holdup. A feed superheated by more than 10° C. can readily lead to excessive local column vapor loading which can lead to the measurable loss of observed staging and/or potential flooding.
A cryogenic rectification process and apparatus for separating air is disclosed in U.S. Pat. No. 6,000,239 in which the operational temperature of the turbine is increased and the superheating within the turbine exhaust is removed through indirect heat transfer. In this patent, the air after having been compressed and purified is divided into two streams. One of the two streams is fully cooled within the main heat exchanger and introduced into the higher pressure column. The other of the two streams is compressed within a booster compressor and then subdivided. A portion of the stream is fully cooled and condensed within the main heat exchanger. Another portion of the stream bypasses the main heat exchanger and is turboexpanded without entering the main heat exchanger. Due to the high inlet temperature, the exhaust stream is superheated and cannot be introduced directly into the distillation column. In this patent, an oxygen-rich liquid stream from the lower pressure column is pumped and then passed into a heat exchanger to indirectly exchange heat with the exhaust stream and thereby remove its superheated state. Thereafter, the oxygen-rich liquid stream is vaporized within the main heat exchanger together to produce a high pressure oxygen product.
In U.S. Pat. No. 6,000,239 an extremely high expansion ratio must be used to enable the exhaust to bypass cooling within the main heat exchanger. Such a turboexpander would be a highly specialized device exhibiting an isentropic expansion efficiency somewhat lower than those typically associated with a more conventional expansion ratio (˜90%). However, since the expansion is conducted at elevated temperature, the turboexpander operates with increased refrigeration-work per unit of mass flow. This fact offsets lower isentropic efficiency and furthermore makes for a more compact process.
Relevant to this discussion is U.S. Pat. No. 3,355,901 that has as its objective the control of the degree of superheating within a turbine exhaust. However, the problem addressed in this patent is in some respects the reverse of the technological problem addressed in U.S. Pat. No. 6,000,239 and the present invention. In U.S. Pat. No. 3,355,901, it is intended to impart a slight degree of superheating to the turbine exhaust rather than removing the superheating from the exhaust. In this patent, a warm portion of a vapor stream to be cooled and introduced into a turboexpander is combined with the cooled vapor stream to ensure a slight degree of superheating in the turbine exhaust to prevent damage to the turbine. The degree of superheat is controlled by adjusting the flow of the cooled vapor stream and therefore, also, the warm portion thereof, to in turn control the temperature at the inlet of the turbine.
This control is effectuated by positioning a temperature sensitive bulb within a line leading to an air separation unit that contains a reference fluid having the same composition of turbine exhaust, for example, air. The bulb is pressurized to the saturation pressure of the reference fluid. The pressure difference between the bulb and pressure measurement taken within the line leading to the air separation unit are compared within a differential pressure transmitter to produce a signal referable to the saturation temperature of the turbine exhaust. In a cascade control scheme, a subsequent controller is used to correct the output signal from the differential pressure transmitter as required and the output of such controller is then sent to a further controller to control the setting of a flow control valve upstream of the turboexpander.
As will be discussed, the present invention provides a method and apparatus related to cryogenic distillation in which a turboexpander is utilized at high temperature to generate a superheated exhaust stream at more conventional expansion ratios to avoid the use of specialized and expensive equipment.